How 60p to 24p conversion works and some experiments.

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dmoreno

Active member
I finally got my hand in some raw 60p footage from the GH1! (Thanks to J Davis for shooting and uploading!)
This gave me the chance to finally test my theory on how this conversion could be made and how good the results would be.
I´m gonna start this thread with a post with a summary I wrote of how the process works in theory.
Then I'm gonna post some actual results from my testing using Vegas and Cineform Neo Scene.
 
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Converting 60 frames to 24 frames is kind of a complicated process since 60 is not a multiple of 24 (converting 60 frames to 30 is very easy because you just drop every other frame or "blend" every 2 frames into one).
Lucky PAL land users can do a perfect conversion from 50p to 25p by simply dropping every other frame. We NTSC people can do a perfect conversion from 60p to 30p. Though, they cannot record footage at real film framerate.

There are two ways to convert 60 frames to 24, none of which is perfect: A. Dropping the extra frames or B. Blending some frames (mixing several frame information into one frame).


A.Frame Dropping

In order to get 24fps from 60 fps by dropping frames you need to keep 24 frames and drop the exceeding 36 frames out of every 60 frames. Dividing all this numbers by 12: You need to keep 2 frames and drop the exceeding 3 frames out of every 5 frames.

The best way to drop those 3 frames out of 5 and keep the motion looking fluid is to keep1, drop1, keep1, drop2, and so on… If we use “O” to represent the kept frames and “_” for the dropped ones you can see the actual temporal separation of the kept frames:

O_O_ _O_O _ _O_O _ _ O_O_ _O_O _ _O_O _ _

This will result on 24 fps footage but with a somewhat awkward motion, resulting from the fact that the lapse between the kept frames is not constant (1 frame one time, 2 frames the next one).

This is the way most NLE's will do the conversion if "frame blending" is disabled (frame interpolation, frame resample, or other names might be used).

If you're gonna use this method, to achieve a film-looking motion blur you should record using the shutter speed closest to 1/48 you can get, that will be 1/60 since it is impossible to use a shutter speed slower than the framerate (as has been explained in other posts using a shutter speed slower than 1/60 when shooting 60p on your GH1 causes the camera to repeat every 2 frames, resulting in 30p instead of 60p). This factor might make this method look less "filmic" to some since the 1/60 shutter is a little bit faster than the standard 1/48 (180 degree shutter) giving the image less motion blur. I guess most people won't notice it at all.

This "frame dropping" method also has the advantage of being less processor intensive and should require less rendering time than frame blending. As I said before, this method won't yield perfect motion due to irregular intervals between kept frames. In shots with little motion, as a “talking head”, it's almost impossible to notice the motion imperfection. In shots with regular speed motion, such as a ball rolling from one side of the screen to the other or a car passing by, the jerkiness becomes more evident but most people won’t be bothered by it.

Actually most of us are used to this "irregular cadences" from watching 24fps footage in our good old NTSC televisions that insert the 3:2 pulldown that distorts the motion in a very similar fashion.

B. Frame blending (frame interpolation, frame resample)

Frame blending is basically “mixing” several frames into one to lower the framerate.

In a 60p to 24p conversion you convert every 60 frames into 24 by blending every 2.5 frames into 1. This works something like this: you set frame A to 40% opacity, frame B to 40% opacity and frame C to 20% opacity to make a new blended frame. Next frame should be frame C 20% opacity, frame D 40% opacity, frame E 40% opacity. And so it goes…

A more advanced way of converting 60 frames to 24 is using an "intelligent" processes that will use the actual 60p frames information to guess how each frame would have been if the footage was actually shot in 24p, this might be called frame interpolation or frame resample.

Any of this methods result in a better motion cadence, due to the regular time separation between the resulting frames. You should consider that using a frame blending process should require longer rendering times and more processing power. Depending on the software implementation of this process, it might result in blurry footage, ghosting or weird artifacts.

If frame blending is gonna be used, footage should be recorded using a shutter of 1/120 in order to get a 1/48 looking motion blur. This, because the "adding" of several frames results in a frame with the added exposure time of the original frames (adding the information of 2.5 frames shot for 1/120th a second will result in a frame with a "time information" of 1/120+1/120+1/120/2 that is exactly 1/48th of second.).
This method is, in theory, capable of yeilding a smoother motion and a simulated shutter very close to 1/48 (if the footage was shot at 1/120, if shot with slower shutters as 1/60 it might result in too much motion blur and soft looking footage).

Conclusions

So, which process is better???? It depends on your taste, the software you plan to use, and the time you want to spend in processing.
In my case, I would use "frame dropping" most of the time, especially in scenes with little motion. Maybe for sports, and things like that, frame blending might result in best looking motion, but it depends on what you prefer: A. Untouched frames with a somewhat jerky motion or B. Perfect motion with artificial frames that probably have artifacts and ghosting

(Sorry for the longest post I've ever seen. Please excuse me if my english is not perfect, it's not my native tongue. Hablo español!!! If my English didn´t disturb you probably the irregular cadence of dropping frames won´t bother you either! :) )[FONT=&quot]
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In order to do my testing I decided to label every frame of the 12 second clip J Davis uploaded
(every frame would have a consecutive number from 0 to 719).
I used Sony Vegas Timecode filter set to "actual frames" to render the clip again with numbered frames so that I could really see what the different conversion methods were really doing. (I had to slow it down to 50% and then speed it to 200% again after labeling since the timecode filter would only label 30 frames per second)
This is the resulting labeled 60p clip in case you want to do your own conversion tests. http://www.produccionesinfinito.com/60p_to_24p/60p_with_labels
 
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The first test I did today was simply dropping the labeled 60p clip on a 720 24p timeline in Vegas.
Vegas, by default, sets clips to "smart resample" that, from what I've seen, uses frame blending every time you use footage with a frame rate different from the timeline or whenever you change clips speed.

I rendered to mp4 720 24p and this is the resulting clip from Vegas frame blending (smart resample) http://www.produccionesinfinito.com/60p_to_24p/60p_to_24p_using_vegas_smart_resample.mp4.

As you will be able to see, especially if you look frame by frame, the resulting footage appears to have some ghosting or an increased motion blur. The motion looks very fluent, but you can really see this are blended frames. It is important to notice that the footage used for this conversion was shot using a 1/60 shutter, had it been shot with a 1/120 shutter the motion blur would have looked, in theory, similar to 1/48 motion blur. Whenever I get my hands on some 1/120 60p footage I will see how that turns out.

Here are some frame grabs from that footage showing the obvious blending going on (look at the blended labels in the lower right corner).

This is the first frame of the converted footage: http://www.produccionesinfinito.com/60p_to_24p/60pframeblend1.png as you can see on the blended label it is using information from frames 0,1,2 and 3 of the original footage.

This is the second frame: http://www.produccionesinfinito.com/60p_to_24p/60pframeblend2.png it appears to have used information from frames 3, 4 and 5.

This another frame showing the increased motion blur/ghosting http://www.produccionesinfinito.com/60p_to_24p/60pframeblendmotionblur.png this frame appears to have blended information from frames 249, 250 and 251. This is the real frame 250 for comparison: http://www.produccionesinfinito.com/60p_to_24p/60pframeblendrealframe.png
60p_to_24p_using_vegas_smart_resample.mp4
 
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The second test I did was setting the clip's switches to "disable resample". From previous experiments I had noticed this would make Vegas simply drop frames.

This is the resulting footage: http://www.produccionesinfinito.com/60p_to_24p/60p_to_24p_using_vegas_disable_resample.mp4

If you look at it frame by frame you will be able to tell there is no blending going on as the frame labels are intact. If you check the labels of the first 10 frames you will see this labels: 0, 2, 5, 7, 10, 12, 15, 17, 20, 22, 25.
Adding a "_" to represent the dropped frames (frames 1,3,4,6,8,9,11,13,14,16,18,19,21,23 and 24) the resulting cadence would look like this:
0_2_ _5_7_ _10_12_ _15_17_ _18_20_22_ _25

This shows Vegas is dropping frames as it should to get the best possible motion when using the frame dropping method (keep 1, drop 1, keep 1, drop 2).

Some other software may use another dropping sequence that could make motion look less fluid (as this 1,2,_,_,_,6,7,_,_,_,11,12) so its good to know it is doing it the right way.

This converted motion, looks a lot like real 24p to me. The motion sometimes looks a little jerky, but I am not sure if its only because the software sometimes stutters during playback. The motion blur looks pretty filmic to me, even though it has a 1/60 shutter as the kept frames from the original footage remain untouched.

I also tried converting 60p to 24p using Cineform Neoscene and it resulted in a dropping frames conversion with the correct expected dropping sequence and motion cadence. This makes me think I could let Cineform handle all my GH1 AVCHD footage and let it remove pulldown from 24p footage and drop frames from 60p footage resulting in 24p only footage that would work pretty well together.

I also tried slowing the labeled 60p footage to 40% speed on a 24p timeline. It resulted in pefect slomo: http://www.produccionesinfinito.com/60p_to_24p/60p_to_24p_slomo40%.mp4. I checked frame by frame and every frame is an untouched frame from the original footage, only played slower. Slomo would look better if the original footage had been shot at a higher shutter speed, ideally 1/120.
 
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Excellent posts - at last I am starting to understand how this stuff works 'under the hood' - thanks!
 
Thanks a lot. I also had the theory in my mind but wanted to see what was really happening in real life!
 
Unless there is no real motion frame blending always leaves ghosting. The only way to have it look good, when it does look good, is interpolation.

Instead of blending the two images together the software needs to look at several consecutive frames and 'decide' what has moved, then calculate the intermediate images.

Needless to say this is slow (think days, not hours), inexact (needs enough data and not too much motion) and if done by someone else, expensive.

Here's one example and I make no claims regarding the usefulness of these programs.

http://www.goodervideo.com/index.html
 
There's a sticky related to this that has several mac workflows for 60p-->24p among other things.
 
For all the people looking for good PC workflow to convert 60p to 24p.
If you use Vegas, it is as easy as dropping the 60p raw clips in a 23.976 fps timeline and editing your video. If you want to have a good conversion, before rendering select all video clips, go to edit menu, switches, disable resample. This will make Vegas drop frames instead of blending them.
My workflow will be a little different because I will be converting everything to Cineform, since I already purchased Neo Scene to extract pulldown from my Canon HV30 24p footage.
I am hoping to use Neo Scene with my GH1 (if I ever get it, I have a preorder in JR) to remove pulldown from 24p footage, convert 60p footage to 24p by dropping frames on the fly while capturing or convert 60p AVCHD to 60p Cineform for faster editing in Vegas in case I want slomo (I edit on a Core2Duo laptop that doesn't handle raw AVCHD that well).
 
Dmoreno, thank you so much man! I only noticed the ghosting when I would PAUSE/PLAY the footage and look. When it played through, I could hardly tell. I will try your other methods and see which I like the best.

Again, thanks!

edit- wow, i just through it in the cineform machine and it came out magnificent! i think i will use cineform as well.
 
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Yes, I think Cineform Neoscene is great investment. Specially for us NTSC users that need to remove pulldown. The codec is also wonderful to work with.
 
Hey dmoreno, I know this is a few months old, but thank you for the precious info! I was looking into this exact topic! "Turning 60p to 24p without slowing down the footage". Thankyou.

I have a quick question, though. How would this work with 60i clips?

Cheers~
 
To my eyes, conforming GH1 720 60p to 24p in vegas 9d by dropping in a 23.97 timeline does not look that great. It is fairly choppy compared to what the 1080 24p looks like.

Neoscene seems to do a better job, but neoscene affects the quality of the footage, adding a slight amount of unwanted noise in detail areas especially in wide shots. I have done a/b tests and the neoscene footage is NOT as clean as the original avchd, even in the HIGH quality mode which takes up ridiculous amounts of hd space.

I really wish Edius neo booster worked.. it seemed to create the highest quality output from the gh1, because it didn't use the neoscene digital intermediate, but it does not handle 108024p files correctly (native)... only the 720 60p.
 
dmoreno said:
I finally got my hand in some raw 60p footage from the GH1! (Thanks to J Davis for shooting and uploading!)
This gave me the chance to finally test my theory on how this conversion could be made and how good the results would be.
I´m gonna start this thread with a post with a summary I wrote of how the process works in theory.
Then I'm gonna post some actual results from my testing using Vegas and Cineform Neo Scene.
Click to expand...
Great idea. My friend and I have our hand on the camera and are looking for 60P slow motion shots. Any advice is helpful, as we are AGDVX people. The Lumix is new to us, but he is a photographer, so that will come into play as well.
 
dmoreno said:
There are two ways to convert 60 frames to 24, none of which is perfect: A. Dropping the extra frames or B. Blending some frames (mixing several frame information into one frame).[FONT=&quot]
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Click to expand...
In addition to frame dropping and frame blending, there is a third option available in Adobe Premiere/After Effects. In the rendering options, you can choose either or both Frame Blending or Motion Blur. If you disable Frame Blending and enable Motion Blur, it will render motion-blurred rather than ghosted images when it extracts 24 frames out of the 60fps. Static, non-panned image details will not be blurred, only the objects that move.
 
Maybe someone else can confirm this, don't have a lot of experience converting 60p to 24p but I shot a documentary pre-gh13 hack. I shot it in 720/60p because I was shooting a lot of hand held and wanted to make sure the footage held up. I am debating whether I will just keep it at 60p or convert everything to 24p for a more "cinematic" look.

I dropped some of the 60p footage into Sony Vegas Pro 10 and it seemed to look pretty good. Not sure if there was an improvement from Vegas Pro 9 to Vegas Pro 10 but can someone else try this and let me know if it looks good to you also. I could be wrong about this.

Thanks!
 
60->24 isnt going to be smooth unless it was originally shot in 24. Doing a reasonable conversion that way would require using interpolated frames to make up for the difference which would make the quality suck. Maybe your audience is too dense to tell, then just go crazy and slip some porn intheretoo.
 
Kind of agree. I am leaning toward just keeping it 60p, as it is a documentary and cinematic frame rate is not he important factor. Given the glass I used I was able to get some nice shallow depth of field along with some color correction. The shots, even in 60p, still love very cinematic.
 
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